Download Climate Change and Ecosystem Services of Florida`s Largest Water

SGEF217
Climate Change and Ecosystem Services of Florida’s
Largest Water Body: Lake Okeechobee1
Karl Havens2
Summary
Future climate change could result in higher temperatures
and greater evaporative water loss in Florida. If these
changes are not compensated for by more rainfall, the state’s
largest water body, Lake Okeechobee, could experience
prolonged periods of very low water levels and catastrophic
loss of its ecosystem services, which are the benefits that
people receive from ecosystems.
as sheet flow, directly hydrating pond apple forests and
sawgrass prairies.
Background
Lake Okeechobee is a large, shallow lake located in the
center of the Florida peninsula, about midway between Orlando and Miami. The natural lake is the largest in Florida
and in the southeastern US. At over 430,000 acres, it is one
of the shallowest large lakes in the world, averaging just 9
feet deep. The lake bottom is uplifted seabed. This seabed
remained dry at the end of the last Ice Age, when sea level
was low and climate was dry in Florida. Even after the ice
age was over, the shallow basin remained dry. Six thousand
years ago, during the Holocene period, peat build-up at the
south end of the lake created a natural dam that ultimately
enabled the lake to form. Up until the early 1900s, the lake
received its water from the meandering Kissimmee River,
from a broad wetland that connected Lake Istokpoga in the
northwest to Lake Okeechobee, and from the wetlands of
Fisheating Creek, which flows into the lake from the west.
Water levels in the lake were highly variable, and at times
water spilled out of the southern end into the Everglades
Figure 1. A photo of Lake Okeechobee, looking out over the western
marsh region to the open waters of the large lake.
Credits: South Florida Water Management District
Because of flooding in the 1920s, 40s and 50s, and a desire
to use land south, east, and west of the lake for agriculture,
a massive project was undertaken by the US Army Corps
of Engineers called the Central and Southern Florida Flood
Control Project. This project involved construction of a
large earthen dike around Lake Okeechobee, with locks
and gates that release water primarily to the east, into
the St. Lucie Estuary, and west, into the Caloosahatchee
Estuary. Smaller canals to the south deliver water to a large
agricultural area called the Everglades Agricultural Area,
1. This document is SGEF217, one of a series of the Sea Grant College Program, UF/IFAS Extension. Original publication date June 2015. Visit the EDIS
website at http://edis.ifas.ufl.edu.
2. Karl Havens, director, Florida Sea Grant College Program, UF/IFAS Extension, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to
individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national
origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office.
U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County
Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.
where sugar cane and vegetables are grown on land that formerly was part of the Everglades. Water from the lake also
hydrates large water conservation areas that help prevent
saltwater intrusion into the water supply of 8 million people
who live along the lower east coast of Florida.
tributaries, including the Kissimmee River. The nutrients
and associated silt have accumulated on the lake bottom,
resulting in a high rate of internal nutrient cycling in the
system. For a detailed overview of the history and evolution
of the lake and the source of this introductory material, see
Aumen and Wetzel (1995) and Havens, Aumen, and Smith
(1996).
Ecosystem Services of Lake
Okeechobee
Ecosystem services include direct benefits such as drinking
water or fish that are caught and sold commercially and
indirect benefits such as control of local rainfall or removal
of nutrients from water. Lake Okeechobee, even in its
modern-day, altered configuration, provides a large number
of ecosystem services in south Florida, which include:
Figure 2. A photo of a fish nest in the shallow water of the Lake
Okeechobee marsh.
Credits: Karl Havens
Figure 3. These maps illustrate the historic and current water flow into
and out of Lake Okeechobee. Green areas are natural wetlands, which
once encircled nearly the entire regional ecosystem; blue arrows are
natural water flows; and red arrows are flows in man-made canals.
Credits: South Florida Water Management District
Water levels in the lake now vary much less than they did
historically, both for safety reasons (risk of levee failure)
and to protect the large marsh that formed inside the levee
on the western side of the lake. This marsh area supports a
diverse plant community and a multi-million-dollar-a-year
sport fishery. Another attribute of the lake that has changed
markedly over the last 100 years is nutrient enrichment and
consequent development of algal blooms. Algal blooms result from nutrient-rich agricultural runoff north of the lake.
Nutrients are delivered to the lake quickly by channelized
•
Habitat for several species of recreational sport fish
•
Habitat for a commercial catfish fishery
•
Habitat for federally endangered Everglades snail kites
•
Water supply for downstream agriculture
•
Water supply for urban areas
•
Cross-state navigation via a canal that runs from the
Atlantic Ocean to the St. Lucie River through the lake
and then via the Caloosahatchee River to the Gulf of
Mexico
•
Flood protection for much of southeast Florida
•
Freshwater that periodically is needed to protect
aquatic plants in the Caloosahatchee River when salt
water moves too far up into that system
•
Freshwater that periodically is needed to push harmful
algae blooms out of the Caloosahatchee River to the
Gulf of Mexico
Optimal and Actual Water Levels in
Lake Okeechobee
The optimal range of water levels in Lake Okeechobee
for providing each of these services differs considerably.
Protection of the marsh habitat is optimized by water levels
measured at the center of the lake that vary from 12 to 15
feet, as identified by Aumen and Wetzel (1995) and Havens
(2002). This range of water level wets the entire marsh habitat in summer and allows it to dry out in winter, permitting
natural fires to burn back thatch and exotic plants. Because
the marsh is critical habitat for fish and birds, this range
Climate Change and Ecosystem Services of Florida’s Largest Water Body: Lake Okeechobee
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also is optimal for recreational fish, commercial fish, and
snail kites.
The optimal variation in lake water levels often does not
occur because of the configuration of the lake in relation
to the surrounding landscape. The watershed of Lake
Okeechobee, which is the land area from which rain water
flows into the lake, is many times the size of the lake itself.
In years with just average summer rainfall, the lake can
receive many times its volume of water as runoff from the
Kissimmee River, Fisheating Creek, and other inflows.
When this happens, the lake can rise to 17 feet or more and
threaten the integrity of the levee. Water then is discharged
at high rates via canals to the St. Lucie and Caloosahatchee
Estuaries, resulting in poor water quality, high loads of
sediment, and adverse impacts to aquatic plants, oysters,
and other aquatic life (Steinman, Havens, and Hornung
2002).
Projected Changes in the Climate
of South Florida
In 2013, the South Florida Water Management District,
Florida Sea Grant (UF/IFAS), the US Geological Survey,
and the Florida Center for Environmental Studies at
Florida Atlantic University performed a scenario analysis
to determine how potential changes in the future climate of
south Florida would affect the hydrology of various parts of
the greater Florida Everglades, including Lake Okeechobee,
the Everglades Water Conservation Areas, and coastal
ecosystems of Florida.
The approach that was used to model what climate might
be like in the future (2060) is explained in a report by
Obeysekera, Barnes, and Nungesser (2015). In short, the
projections of 15 different climate models gave average
estimates of a 3oF increase in temperature, a rise in evapotranspiration proportional to the temperature increase, and
a rise in sea level of 1.5 feet. Evapotranspiration is loss of
water to the atmosphere by evaporation and from plants in
a process called transpiration. One major factor controlling
the rate of evapotranspiration is temperature. The climate
models did not agree on what will happen with future
rainfall. Some models predicted more rain, some predicted
less rain, and others predicted no change. For this exercise,
therefore, we used both plus and minus 10 percent rainfall
as possible future conditions.
Effects on Water Levels in Lake
Okeechobee
Effects of the projected changes in climate on water levels
in Lake Okeechobee were determined using a regional
hydrologic model that the South Florida Water Management District developed in the 1990s and used to evaluate
alternative plans for restoring the Everglades. This model,
known as the South Florida Water Management Model
(SFWMD 2005), considers water entering the regional
system by rainfall, water moving over the surface and in
the ground, water taken from the system for consumptive
uses (e.g., crop and lawn irrigation and drinking water),
water lost to the ocean, and water lost to the atmosphere by
evapotranspiration.
Figure 4. A cross-section through Lake Okeechobee (highly exaggerated in the vertical dimension) showing three major zones of the lake: a
deep pelagic or open-water zone, a shallow littoral or marsh zone, and a zone in between, lying on a shelf of intermediate depth called the
near-shore zone. The marsh supports emergent plants like cattail, willow, and sawgrass. The near-shore zone supports submerged (underwater)
and floating-leaved plants like tapegrass, pondweed, and water lily. The pelagic zone is too deep to support any plants. The two-ended arrow
illustrates an optimal situation of rising and falling water levels between summer and winter that wets and dries the marsh zone, i.e. 12- to 15foot range of depth, measured at mid-lake.
Credits: Karl Havens
Climate Change and Ecosystem Services of Florida’s Largest Water Body: Lake Okeechobee
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The model simulates inputs and outputs of water to and
from Lake Okeechobee, respectively, and generates a time
series of projected water levels based on a historic time
series of rainfall. In this case, the time series was 1965 to
2005. Model runs were done for different possible futures.
These were compared to a future condition with no change
in any of the climate variables. From here on the following
abbreviations will be used: ET (evapotranspiration) and
RF (rainfall). This model asks “if climate conditions were
altered in this manner what would the water depths in Lake
Okeechobee have looked like over the period 1965 to 2005,
compared to what actually occurred?”
This was done with these climate conditions:
• A future with increased temperature and ET with no
change in rainfall [+ET]
• A future with increased temperature and ET and 10
percent more rainfall [+RF+ET]
• A future with increased temperature and ET and 10
percent less rainfall [-RF+ET]
For coastal ecosystems in south Florida, such as coastal
estuaries, sea-level rise is directly important, but for Lake
Okeechobee, located at the center of the state and surrounded by a levee, effects of sea-level rise had no influence
on future conditions in the model runs.
In Lake Okeechobee, the future model condition with 10
percent more rainfall balanced the effects of higher temperature, and a 40-year time series of water levels showed
no noticeable change (Havens and Steinman 2015). In the
future model condition with no change in rainfall and only
increased temperature and evapotranspiration, water levels
in the lake dropped markedly. Instead of having lake levels
in the optimal depth zone of 12 to 15 feet about 60 percent
of the time (future with no climate change), optimal depth
conditions were projected to prevail just 40 percent of the
time. In the future model condition with higher temperature and evapotranspiration and a 10 percent decline in
rainfall, severe low lake levels occurred, at times reaching
just 5 feet, a point at which a person could nearly walk
across the lake. There were periods lasting several years
when the lake was below 8 feet and the entire marsh region
and near-shore region of submerged plants would be dry. In
that hot/dry future, the lake is in its optimal depth zone of
12 to 15 feet only 15 percent of the time.
Differences between scenarios in the year 2060 are most
evident if we look at water levels in the lake using visual
aids (color-coded maps of water depths). At 15 feet, the
lake has standing water right up to the edge of the levee,
including shallow water over the entire western marsh
region. At 12 feet, the marsh is dry, but water still covers a
zone between the marsh and open water of the lake, an area
that is important for submerged plants and fish nurseries.
As noted, the range between these two extremes is optimal
for health of the lake (Aumen and Wetzel 1995, Havens
2002). In contrast, in the hot/dry 2060 scenario, the lake
is reduced to a small area of very shallow water, and all of
the critical habitat areas are bone dry. There were no future
scenarios with more water in the lake; only status quo if it
rains more in 2060, or lower lake levels if rainfall stays the
same or declines.
Figure 5. Water levels in Lake Okeechobee from 1965 to 2005, simulated by the South Florida Water Management Model under different future
climate conditions. Depth is on the vertical axis and year on the horizontal axis. The two horizontal lines (at 12 and 15 feet) bound the optimal
zone of lake levels for a healthy marsh, submerged plants, and fisheries.
BASE = future with no changes in climate; +ET = future with no change in rainfall (RF) and higher temperature and evapotranspiration (ET):
+RF+ET = future with more rainfall and higher temperature and ET, which counter-balance one another and so the line lies right on the line for
the BASE; -RF+ET = hot/dry future situation with less RF and greater temperatures and ET.
Credits: Havens and Steinman, 2015
Climate Change and Ecosystem Services of Florida’s Largest Water Body: Lake Okeechobee
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Effects on Ecosystem Services
To assess how these changes in future water levels might
affect ecosystem services, consider again that the lake has
distinct zones, each of which provides different services
to society. The western marsh zone, the shallowest part of
the lake, provides important feeding and nesting areas for
migratory birds, fish, and the Everglades snail kite. When
lake stage is high, near the 15-foot level, the marsh is also
a prime area for bass fishing and bird watching. In future
scenarios with the same or less rainfall, the marsh would
be dry for long periods of time, woody plants would likely
replace aquatic plants, and these services would largely be
lost. It is uncertain if such services could recover in short
intervals when lake level is higher because it might take
years for aquatic plants to become re-established after
woody plants are flooded and die.
The near-shore area of the lake, which occurs between
the marsh and the open-water central area, supports
submerged plants and is a prime habitat for fish, fishing,
boating, and bird watching. This area, too, is dry for long
periods of time in the scenarios with the same or less rain
in the future. In fact, the periods during which this area
is completely dry may be so frequent and long-lasting
(especially in the future with less rainfall and increased
temperature) that the lake no longer can support any of
these services. It is unclear whether the plants and the
habitat they provide could shift to deeper water—probably
not. The area where there is water in the right two panels
of Figure 6 coincides almost exactly with the area of the
lake that has soft, unconsolidated mud on the bottom, and
plants cannot take root in such loose, constantly shifting
sediment.
In a future with a very shallow lake, wind would likely
suspend the mud continually, resulting in highly turbid
water with little or no light penetrating to the lake bottom—and, therefore, no plants. That low-water future
would see a lake with extremely high concentrations of
the nutrient phosphorus in the water because suspended
bottom sediments are rich in phosphorus (Havens 2007).
High levels of phosphorus might stimulate blooms of toxic
blue-green algae if there were sufficient light to allow them
to grow. Such blooms might be most dense around the edge
of the area covered with water, another reason that it is
unlikely that plants could grow on the bottom—because of
intense shading by algae.
Direct human services, including navigation and water
supply, would be severely impacted by a very low Lake
Okeechobee. There are two US Army Corps of Engineers
navigation routes across the lake: (1) a channel that crosses
the lake just south of its center, and (2) another that goes
around the southern end of the lake in a “rim canal.”
Havens and Steinman (2015) calculated that in a future
with just higher temperature and evapotranspiration,
navigation might not be affected. They concluded, however,
that in a hot/dry future with less rain, higher temperature
and more evapotranspiration, the cross-lake route could be
navigated by a small boat (1.5-foot draft) just 72 percent of
the time and the rim canal route just 50percent of the time.
Figure 6. Depth contour maps showing the extent of water in Lake Okeechobee in its optimal range (left two panels) and under extreme drought
conditions, as might occur in a hot/dry future (right two panels). Colors correspond generally to water depths, with dark blue about 15 ft and
orange less than a foot (Source: South Florida Water Management District).
Credits: South Florida Water Management District
Climate Change and Ecosystem Services of Florida’s Largest Water Body: Lake Okeechobee
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In the hot/dry 2060 model scenario, there were reductions
in water supply to agricultural areas downstream of the
lake, where there are vast farms of sugar cane, vegetables
and citrus. Although it is not possible to project exact
impacts, we know that in 2000-01, when the lake dropped
below 9 feet in just one year, cutbacks in water delivery led
to large-scale crop losses and severe restrictions on water
use in the major metropolitan areas of southeast Florida.
The tree and ornamental plant industry lost 2,000 jobs and
the negative direct-value-added impact of the drought on
agriculture was estimated to have been $40 million dollars
(Hodges and Haydu 2003).
In that drought, water was high enough to remain in the
rim canal at the south end of the lake, where it was pumped
out of the lake for human uses. At the lower lake levels in
the right two panels of Figure 6, water could be miles away
from the lake’s edge, and it might be impossible to pump for
human uses.
Lake Okeechobee is an important water supply to downstream ecosystems. Regional-level planning to restore the
Everglades assumes a large amount of water will flow from
the lake to that wetland ecosystem. This could not happen
in the hot/dry future scenario. Nor could there be deliveries
of water to the Caloosahatchee Estuary, which are needed
periodically to reduce salinity that impacts submerged
plants and water intakes for coastal cities. This water is also
needed to push blue-green algae blooms from the Caloosahatchee River to the sea.
Remedies
On a global scale, a large reduction in carbon emissions or
development of technologies that sequester carbon from
the atmosphere might slow or stop climate change. These
options are, however, beyond the scope of this fact sheet.
Assuming that the future is hotter, it is reasonable to expect
lower water levels in Lake Okeechobee and loss of services.
The most recent report of the Intergovernmental Panel on
Climate Change (IPCC 2014) indicates that in the future,
droughts might last longer, and when rainfall does occur,
it might be delivered in shorter, more intense events. If
that happens, the outcomes for Lake Okeechobee would
be much worse than projected here because the model
assumed that the temporal pattern of rainfall in the future
will mimic that of the past.
One option for establishing a secure water supply might
be to construct a large reservoir. It could be used to store
water when it rains, and make that water available when it
is dry. The problem with that approach is the tremendous
volume needed. To compensate for the difference in water
storage between the hot/dry future and the base condition,
Havens and Steinman (2015) calculated it would require
a 7-foot-deep reservoir with an area of 220,000 acres to
accommodate just a single year’s storage, because each
year such a reservoir in south Florida would lose about 60
percent of its water to evaporation.
Such a solution would not be helpful in a drought that lasts
3, 5, or 7 years. At this time it is not clear if the ecosystem
services provided by Lake Okeechobee can be protected if
climate change in future decades includes both increased
temperature and less rainfall. The worst-case scenario
underscores the fact that some changes caused by climate
change are so extreme as to preclude adaptation. The larger
goal of reducing the rate of climate change by cutting
greenhouse gas emissions may in some cases be the only
solution to avoid catastrophic outcomes.
Literature Cited
Aumen, N.G., and R.G. Wetzel. 1995. Ecological studies on
the littoral and pelagic systems of Lake Okeechobee, Florida
(USA). Archiv für Hydrobiologie, Advances in Limnology, 45,
356 pp.
Havens, K.E. 2002. Development and application of hydrologic restoration goals for a large subtropical lake. Lake and
Reservoir Management 18: 285–292.
Havens, K.E. 2007. Phosphorus dynamics at multiple time
scales in the pelagic zone of a large shallow lake in Florida,
USA. Hydrobiologia 581: 25–42.
Havens, K.E., N.G. Aumen, and V.H. Smith. 1996. Rapid
ecological changes in a large subtropical lake undergoing
cultural eutrophication. Ambio 25: 150–155.
Havens, K.E., and A.D. Steinman. 2015. Ecological
responses of a large shallow lake (Lake Okeechobee,
Florida) to climate change and future hydrologic regimes.
Environmental Management 55:763–775.
Hodges, A.W., and J.J. Haydu. 2003. Economic impacts of
drought on the Florida environmental horticultural industry.
University of Florida Institute of Food and Agricultural
Sciences, Gainesville, FL. http://edis.ifas.ufl.edu/fe385.
Obeysekera, J., J. Barnes, and M. Nungesser. 2015. Climate
sensitivity runs and regional hydrologic modeling for
predicting the response of the greater Florida Everglades
Climate Change and Ecosystem Services of Florida’s Largest Water Body: Lake Okeechobee
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ecosystem to climate change. Environmental Management
55: 749–762.
SFWMD. 2005. Documentation of the South Florida Water
Management Model Version 5.5. South Florida Water
Management District, West Palm Beach, FL.
Steinman, A.D., K.E. Havens, and L. Hornung. 2002. The
managed recession of Lake Okeechobee, Florida: integrating science and natural resource management. Conservation Ecology 6 (2), Online.
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